[aprssig] FW: PHG part II

Robert Bruninga bruninga at usna.edu
Tue Jan 13 13:16:24 EST 2009

> There's also Bob's "mobile range" document at:
> http://aprs.org/symbols/PHGmobile.txt
>> And the actual range is a darn site more 
>> than 5 miles, that I know for sure.

In defense:

My experience remains "for general mobile operations" that the
average range for decoding DIRECT packets mobile-to-mobile is 5
miles or so, and mostly less. (50W mobiles)

Sure, for a base station with 20 mile range, then you can hear
the mobile further.  The key word here is mobile-to-mobile
(direct) and *average* ...

Since I run voice alert all the time, it is very easy to build
up this experience.  I can hear them sometimes beyond 5 miles,
but cannot decode them.  In fact, I typicallly don't decode
until 3 miles or less.  My area is relatively flat eastern USA
with gentle hills on the order of 100 feet or so...  But if
there are digi's nearby, you cannot really tell if you are
decoding direct or via the digi.

My earliest experience with this was back when there were few
digis along the 500 miles from my QTH to the annual DAYTON
hamvention.  THis was long before voice alert...

But then all the travelers running APRS would appear and
disappear (since mostly direct) and 5 miles was even optomistic.
This includes going over the Appalachian mountains when there
were some 1000+ foot rises and valleys.

But even in those extreme heights, the other guy was often in a
valley when I was high, or he was high when I was low.
Probabilities of both being high at the same time were rare.

> The range calculation formula is in the 
> specification. It's nowhere near 
> reality, and it's not trying to be...

There is the most important point.  What it is trying for is
CONSISTENT presentation of "relative" performance.  Bigger is
better.  Smaller is worse.  And if we all see the same relative
presentation, then we all know how well different stations will
preform in the RF network relative to each other.

It all has to do with mobile flutter.  Flutter can be +/- 30 dB
while moving, and so even if you say on average it is +/- 6 dB,
then that cuts the range immediately in half compared to a base

Just some background.
Bob, Wb4APR

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